This invention relates to materials useful for enhancing the browning, crisping, flavor and aroma of foods cooked in microwave ovens.
Food preparation and cooking by means of microwave energy has, in recent years, become widely practiced as convenient and energy efficient. Microwave cooking of precooked and uncooked food products has traditionally produced bland-appearing and soggy meats and pastry goods. To alleviate this problem and aid the browning and crispening of the surface of a cooked food item, there have been developed a number of packaging materials specially adapted for use in microwave cooking. Many such known packaging materials incorporate a microwave susceptor material, i.e., a material capable of absorbing the electric or magnetic portion of the microwave field energy to convert that energy to heat.
U.S. Pat. No. 4,267,420 to Brastad discloses a packaging material which is a plastic film or other dielectric substrate having a thin semiconducting coating. A food item is wrapped in the coated film so that the film conforms to a substantial surface portion of the food item. On exposure to microwave energy, the film converts some of that energy into heat which is transmitted directly to the surface portion so that a browning and/or crispening is achieved.
U.S. Pat. No. 4,518,651 to Wolfe discloses flexible composite materials exhibiting controlled absorption of microwave energy comprising a porous dielectric substrate coated with electrically conductive particles, such as particulate carbon, in a thermoplastic dielectric matrix. The porous substrate is a sheet or web material, usually paper or paperboard. The patent implies that the porosity of the substrate is necessary so that the susceptor/thermoplastic matrix is adequately absorbed.
U.S. Pat. No. 4,434,197 to Petriello et al. discloses a flexible multi-layer structure having at least one layer colored with a pigment and/or energy absorber with the outer two layers consisting of pure polytetrafluoroethylene to provide a food contacting surface. Disclosed as suitable energy absorbers are colloidal graphite, carbon and ferrous oxide.
U.S. Pat. No. 4,230,924 to Brastad et al. discloses a flexible wrapping sheet of dielectric material, such as polyester or paperboard, capable of conforming to at least a portion of the shape of a food article, and having a flexible metallic coating thereon. The coating, e.g., of aluminum, chromium, tin oxide, silver or gold, converts a portion of microwave energy into thermal energy so as to brown or crispen that portion of the food adjacent thereto.
The above-cited patents all disclose flexible materials used for wrapping around a food item to achieve browning and crispening during microwave cooking. An alternate approach was suggested in U.S. Pat. No. 4,190,757 to Turpin et al. This patent discloses a paperboard carton having a lossy microwave energy absorber which becomes hot when exposed to microwave radiation. The package also preferably includes a shield, e.g., a metal screen or a metal foil cover having holes therein, to reduce by a controlled amount the direct transmission of microwave energy into the food product. The patent notes that, as heating occurs, moisture vapor and steam is vented through the openings in the shield, thereby maximizing the opportunity for moisture to be driven out of the food product and for the food product to become crisp.
Another patent which recognizes the desirability of providing in microwave packaging materials a means for removing liquid by-products is U.S. Pat. No. 4,390,554 to Levinson. Unlike the preceding patents, however the goal of the '554 patent was not to achieve browning and crispening of a food but to overcome spot, selective and edge heating. Also unlike the preceding patents, the packaging system of the '554 patent does not utilize a microwave susceptor material. In the disclosed packaging system, a food item is enclosed in and contacted by a perforated plastic film which in turn is enclosed in a microwave-permeable, water and food by-product absorptive material, all of which are enclosed by a microwave-permeable, liquid-barrier plastic film, all of which are enclosed by a microwave-permeable, heat-insulating material. The absorptive material absorbs liquid escaping during cooking and then itself becomes microwave absorptive, reducing the amount of microwave energy reaching certain areas of the food.
As the above-mentioned patents indicate, there has been no shortage of proposed packaging materials for ameliorating the problems inherent in microwave cooking. Despite all of these efforts, and a number of packaging materials currently available for packaging foods for microwave cooking, it is generally recognized in the trade that certain types of foods are extremely difficult to cook satisfactorily by microwave. These foods are foods which should ideally have a browned or crispened exterior and a moist interior such as egg rolls, fish sticks, french fries, fried chicken and dough-type products. The present invention relates to new packaging materials which can be used to package a variety of foods for microwave cooking, which can enhance the crisping, browning, flavor and aroma of the packaged foods when cooked by microwave without substantially lengthening the required cooking time, and are especially useful for packaging and cooking the aforementioned "difficult" foods.